Devices and methods for ultra-wideband communications

ABSTRACT

A method for generating an ultra-wideband communications signal is described. The method includes generating a piecewise linear ultra-wideband baseband signal comprising at least one pulse, based on an inputted data signal; generating a carrier tone having a carrier frequency suitable for wireless transmission; and upconverting the baseband signal with the carrier tone to the carrier frequency. A method for interpreting a received ultra-wideband communications signal, the signal having a center frequency in the RF domain, is also described. The method includes generating at least one local signal template, synchronized with the received ultra-wideband communications signal and having substantially the same center frequency; correlating the received ultra-wideband communications signal with each of the local signal templates in the analog domain, obtaining at least one ultra-wideband baseband signal; and interpreting the at least one ultra-wideband baseband signal to generate a data signal. Devices for implementing these methods are also described.

RELATED APPLICATIONS

The present patent application claims priority under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application Ser. No. 60/603,128, which wasfiled Aug. 20, 2004. The present patent application also claims priorityunder 35 U.S.C. § 119(b) to European Patent Application No. EP05447023.2 filed on Feb. 11, 2005, and European Patent Application No.EP 05447075.2 filed on Apr. 4, 2005. The full disclosures of U.S.Provisional Patent Application Ser. No. 60/603,128, European PatentApplication No. EP 05447023.2, and European Patent Application No. EP05447075.2 are incorporated herein by reference.

FIELD

The present invention relates generally to Ultra-Wideband (UWB)communications, and more particularly, relates to impulse-based UWBsystems (also called IR-UWB for Impulse Radio).

BACKGROUND

Traditional approaches generally use specialized circuits to generateUWB pulses. Traditional IR-UWB systems generally rely on specificcomponents, such as step-recovery diodes or external inductor coils. Acurrent discharge through these components generates a pulse in theRadio Frequency (RF) domain. The pulse is sent through the air aftersome filtering from the antenna or other components inserted in thechain.

This traditional approach has the disadvantage that only a fixed type ofpulse can be generated. Therefore, it would be beneficial to provide amethod for generating an UWB communications signal that is moreflexible.

SUMMARY

A system and method for UWB communications is described. The system andmethod combine baseband signal processing and up-/down-conversion in thefield of UWB communications. On a transmitter side, a piecewise linearultra-wideband baseband signal is generated, which is then upconvertedto the RF domain by mixing with a carrier tone of the desired RFfrequency. The piecewise linear ultra-wideband baseband signal is asignal that includes at least one pulse.

The term “piecewise linear” means that a graph of the signal is asequence of substantially linear parts. An alternative definition isthat the first derivative of the graph of the signal is non-smooth.

The term “ultra-wideband signal” means, according to FCC regulations, asignal having a bandwidth of at least 20% of the center frequency or atleast 500 MHz, and a center frequency between 3 and 10 GHz. The 5-6 GHzband may be excluded in practice, since this band is used up by WLANcommunication.

By splitting the baseband signal from the RF domain, more flexibilitycan be achieved in the system. The shape of the pulse, thereby itsbandwidth, and the central frequency around which the pulse will beplaced in the spectrum can be separately adapted from each other. Theseparameters can be tuned separately in the final circuit. This is nottrue for traditional IR-UWB systems, which can only produce a fixed typeof pulse. For example, a pulser can be made that allows separate tuningof bandwidth from 500 MHz to 2 GHz, and central frequency from 3 GHz to5 GHz.

In a method for generating an ultra-wideband communications signal,according to an example, the carrier tone is preferably generated bymeans of an oscillator that is switched off between two pulses. In thisway, power consumption may be minimized. The oscillator is preferably aring oscillator due to the ring oscillator's fast start-upcharacteristics.

The pulse or envelope that is used for the baseband signal is preferablytriangular because a triangular form is easy to generate. However, allother piecewise linear pulse forms known to a person skilled in the art,such as rectangular or trapezoidal forms, may also be used.

A pulser (i.e., a central part of the transmitter) includes anoscillator that generates the carrier tone of the desired centerfrequency, a signal generator for generating the piecewise linearultra-wideband baseband signal (herein also called baseband envelope),and a mixer for upconverting the baseband signal to the RF domain bymeans of the carrier tone.

The signal generator may include a pulse position modulator for pulseposition modulation. A pulse shaping circuit may be used to tune theshape of the pulse. In order to reduce the power consumption of thecircuit in very low-duty-cycle IR-UWB (more time is spent between pulsesthan during pulses), a fast start-up ring oscillator is used. The ringoscillator is started and stopped for each pulse, using a specialbiasing to speed-up its starting phase. The oscillator output is thenmixed with the envelope (the baseband signal with discrete pulses) forupconversion to the RF domain.

Other modulation techniques known in the art may also be employed, suchas differential binary phase-shift keying (BPSK) modulation. By creatingan imbalance inside the ring oscillator circuit, the start-up of thering oscillator is enforced with an initial condition. If the imbalanceis inverted, the ring oscillator may start-up in the opposite phase.Differential phase modulation (e.g., BPSK) may then be performed bychoosing the sign of the imbalance depending on the inputted datasignal.

Moreover, the pulser can be more easily integrated than standardtechnology. Specific external or exotic components are not required, sothat the pulser can be integrated into standard digital CMOS withoutexternal components.

On receiver side, a received UWB communications signal in the RF domainis interpreted as follows for retrieving the original data signal. Aplurality of local signal templates is generated in synchronization withand having substantially the same center frequency as the receivedsignal. The received UWB communications signal is then correlated withthe templates, which are phase-shifted with respect to each other, inthe analog domain, and the data signal is retrieved by sampling theoutput of the correlators. The correlation in the analog domain has theadvantage that digital operations for retrieving the data signal arerelaxed due to the lower speed of operation.

Furthermore, a more simple synchronization on the envelope can beachieved, which is sufficient for non-coherent orthogonal pulse positionmodulations, while other systems have to synchronize on the full RFpulse. The required accuracy is largely reduced in this example, whichcan make a big difference as synchronization is known to be abottle-neck in UWB systems.

The correlation preferably comprises the mixing of both signals and theintegration of the mixed signal. However, other correlation techniquesknown to the person skilled in the art may also be used.

The signal templates are preferably generated by switching a ringoscillator on and off, in synchronization with the received UWBtelecommunications signal. This has the advantage of being easy toimplement, while energy loss as a result of a possible synchronizationerror is kept at a minimum. The signal templates may also be implementedin other ways, such as in a similar way as the pulse generation on thetransmitter side in a case in which a strong correlation overlap isdesired.

In one example, two phase-shifted local signal templates are generatedin which the phase shift between the two signal templates is maintainedat substantially 90°. This leads to a correlation in quadrature, whichrelaxes the timing constraints on the local signal templates. In anotherexample, three branches are used, which enables the correlation by meansof three signal templates with 120° phase offset between them. This issimilar to the quadrature correlation. Furthermore, the interpretationcan be based on any number of branches with any phase shift between thelocal signal templates.

In the above described examples, the detection can be based onnon-coherent envelope detection if an orthogonal modulation, such aspulse position modulation (PPM), is used, combining the energy of thetwo/three branches, or coherent detection if BPSK modulation is used,coherently recombining the two/three phases. This choice is not as easyfor traditional (non carrier-based) IR-UWB systems. For thesetraditional IR-UWB systems, the best option is to convert everything tothe digital domain where the processing takes place. However, this leadsto higher power consumption due to high-speed ADCs and heavy digitalprocessing.

Based on the above described concept of interpretation of a received UWBcommunications signal, a suitable receiver includes a signal generatorfor generating the plurality of signal template, a timing circuit actingon the signal generator for synchronization, a plurality of analogcorrelators (one per template) for correlating the UWB communicationssignal with the signal templates, and (digital) interpretation means(e.g., a baseband detection circuit) for finally deriving a data signal.The receiver may also include other components.

Like the pulser on transmitter side, the “interpreter” on receiver sidecan also be implemented in CMOS.

Each of the analog correlators preferably includes a mixer and an analogintegrator downstream of the mixer. The mixer receives as inputs thereceived UWB communications signal and one of the signal templates. Thesignal generator preferably includes a ring oscillator, which isswitched on and off by the timing circuit. In the case of a quadraturecorrelation, the receiver preferably includes two analog correlators anda phase shifter, which generates a second signal template byphase-shifting a first signal template over substantially 90°. In thecase of a three-branch correlation, the receiver preferably comprisesthree analog correlators and phase shifting means for maintaining thephase shift between three signal templates at approximately 120°. Asmentioned above, a different number of branches and different phaseshifts between the signal templates are also possible.

Precise timing generation that is re-usable in the various parts of anUWB transmitter and an UWB receiver is beneficial. Furthermore, it isdesirable that the timing generation solution be suited for CMOSimplementation and does not consume too much power. The precise timinggeneration may be realized by an arrangement of a current source, afirst capacitor, and a comparator.

In an example, a pulse position modulating circuit is provided that usesthe precise timing generation mechanism, by providing a secondcapacitor, switchable with the first capacitor, in accordance with theinput data of the pulse position modulating circuit.

In an example, a pulse shaping circuit is provided that uses the precisetiming generation mechanism, by providing means to charge and dischargethe first capacitor by feeding back an output of the comparator.

In an example, an oscillator activation circuit is provided, which isbased on essentially the same principle as the pulse position modulatingcircuit. In a device in which both the pulse shaping circuit and theoscillator activation circuit are present, a fixed relationship betweenthe current sources of the circuits is set.

These as well as other aspects and advantages will become apparent tothose of ordinary skill in the art by reading the following detaileddescription, with reference where appropriate to the accompanyingdrawings. Further, it is understood that this summary is merely anexample and is not intended to limit the scope of the invention asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Presently preferred embodiments are described below in conjunction withthe appended drawing figures, wherein like reference numerals refer tolike elements in the various figures, and wherein:

FIG. 1 is a graph depicting spectrum specification definitions;

FIG. 2 is a graph depicting spectrum requirements of UWB devices due toregulations;

FIG. 3 is a block diagram of a UWB transmitter architecture, accordingto an example;

FIG. 4 is a circuit diagram of a timing generation circuit, according toan example;

FIG. 5 is a circuit diagram of a pulse position modulator, according toan example;

FIG. 6 is a graph depicting simulation results of the operation of thepulse position modulator depicted in FIG. 5;

FIG. 7 is a circuit diagram of a pulse shaping circuit, according to anexample;

FIG. 8 is a graph depicting simulation results of the operation of thepulse shaping circuit depicted in FIG. 7;

FIG. 9 is a graph depicting simulation results on the ring activationcircuit;

FIG. 10 is a circuit diagram of a mixer circuit, according to anexample;

FIG. 11 is a graph depicting simulation results of the operation of thering oscillator;

FIG. 12 is a graph depicting tunability of the ring oscillator,according to an example;

FIG. 13 is a layout of the UWB transmitter depicted in FIG. 3, accordingto an example;

FIG. 14 is a circuit diagram of a UWB receiver, according to an example;

FIG. 15 is a graph depicting drift between received UWB signal andsignal template; and

FIG. 16 is a circuit diagram of a UWB quadrature receiver, according toan example.

DETAILED DESCRIPTION

The UWB technology is a potential solution for an ultra-low powerimplementation for wireless communication. Indeed, the heavy duty cyclenature of the signals allows most of the system to be turned off betweenthe UWB pulses. The energy consumption is calculated by referring to thetransmission of a single pulse. Since several pulses are required totransmit one bit of information, the energy per bit is given by thenumber of pulses (Nppb) required for one bit times the energy per pulse(Epp). This last quantity is important specification for a UWB pulser,the former being more related to the link budget calculations.

As an example, if one hundred pulses are required to transmit one bit ofinformation, and the targeted energy per bit is 10 nJ per bit, thetargeted energy consumption per pulse is then 100 pJ per pulse. Ofcourse, between the pulses, there is also some energy consumption (i.e.,standby power, Pstby) that needs to be taken into account in the finalcalculation.

Some other parameters are related to the signal itself. Theinstantaneous power, directly related to the amplitude (Ap) of thepulse, is defined as the output power (Pout). Pout is defined asfollows:${P_{out}\lbrack{dBm}\rbrack} = {10\quad{{\log( {\frac{A_{p}^{2}}{2.50} \cdot \frac{1}{1\quad{mW}}} )}.}}$Pout is used in the link budget calculation.

Another parameter is the bandwidth (Bw) of the pulses. This quantity isdefined by the −10 dB points referred to the maximum point in thespectrum of the signal. The maximum point in the spectrum is shown inFIG. 1.

The system is based on modulated pulses. The bandwidth of the pulses iscentered around a carrier frequency (Fc), which is defined as the middlepoint of the bandwidth. Note that this carrier frequency is usuallydifferent than the maximum frequency inside this bandwidth.

The FCC regulation in the United States requires UWB communication tooperate under precise conditions. Other regulatory bodies in the worldare still evaluating the impact of UWB systems on other existingcommunication systems. According to the FCC, a UWB signal is defined asa signal for which its −10 dB bandwidth is higher or equal to 20% of itscenter frequency, or a signal for which its −10 dB bandwidth is higheror equal to 500 MHz. Since the UWB power should not harm other existingcommunication systems, a maximum average power spectral density isdefined as −41 dBm/MHz. To avoid interference with the low power signalsof GPS systems, a lower spectral density is defined for a signal below3.1 GHz. The spectrum mask is given in FIG. 2.

However, UWB operation between 3.1 GHz and 10.6 GHz results in anoverlap with the IEEE 802.11a WLAN standard, which operates in the ISMband between 5 GHz and 6 GHz. In order to prevent interference with theIEEE 802.11a WLAN standard, operation between 5 GHz and 6 GHz isavoided. If 0.18 um CMOS technology is used, it may be difficult tooperate at low power consumption between 6 GHz and 10 GHz. As a result,the UWB system is preferably designed to operate between 3.1 GHz and 5GHz.

The pulse rate is a power consumption compromise between pulse dutycycling and clock generation power. High clock rates allow a loweroutput power, thus lower power consumption, but higher clock powerconsumption. Lower clock rates result in higher output power, but lowerclock consumption. First estimates of the optimum pulse rates result ina clock rate around a few tens of MHz. The optimum pulse rate ispreferably around a few tens of MHz. The pulse rate is preferablytunable around a predetermined value. The pulse repetition time is theinverse of the clock rate (=pulse rate). A typical value of the pulserepetition time is in the order of 100 ns.

A periodic signal gives rise to spikes in the spectrum, which may resultin an infringement of the FCC rule. Randomization in the repetition ofthe pulses can be implemented to smooth these spikes. Pulse positionmodulation is the most efficient means to smooth the spectrum. Sincepulse position modulation implementation is not too complex, thismodulation approach is preferably chosen.

The pulse position (Tpos) relative to a precise clocking instant definesthe value of the data information. The specification for this valueresults from the detection method used in the receiver. For thedetection to be optimal, position of the pulses should not overlap. Thelongest pulse duration occurs for the 500 MHz bandwidth, which is in theorder of 2-4 ns depending on the pulse shape. Twice this value will thenguaranty non-overlapping symbols. A maximum for the pulse positionmodulation time is given by half the repetition time (Trep/2˜50 ns) inorder to distinguish a symbol from the next one.

The output power required is related to the pulse repetition rate.Indeed, since the FCC regulation restricts the average output power,increasing the pulse rate results in a lower pulse output power. Thecalculations of output power result from link budget analysis. Thesehave been roughly done resulting in a required output power of around 0dBm.

For UWB communication, the standby power is an important parameter toreduce as much as possible. The standby power for typical low powercommunication systems for similar applications is around 10 nJ/bit. Astandby power target of 1 nJ/bit seems possible with UWB, whichdemonstrates the low power capabilities of UWB.

In summary, the preferred pulser specifications are given in thefollowing table: Parameter Specification Frequency range Fc 3 GHz-5 GHzBandwidth Bw 500 MHz-2 GHz Pulse repetition time Trep >50 ns Modulationtype Pulse position PPM timing position Tpos 4 ns-25 ns Output powerPout 0 dBm Energy consumption per pulse Epp 10 pJ Standby power Pstby<<<

The system is thus designed for transferring digital data intoposition-modulated pulses with a tunable bandwidth of at least 500 MHzcentered on a carrier between 3 GHz and 5 GHz. A block diagram of a UWBtransmitter 300 is depicted in FIG. 3.

The UWB transmitter 300 is subdivided into five basic functions,including a pulse position modulator 302, a pulse shaping circuit 304, aring activation circuit 306, a ring oscillator 308, and an antennaadaptation circuit 312. The pulse position modulator 302 enables thepulse shaping circuit 304 and the ring activation circuit 306 at avariable time instant with respect to a clock. The pulse shaping block304 creates an envelope for a pulse. In this example, the pulse is atriangle with a tunable duration.

The ring activation circuit 306 creates a ring activation signal. Thering activation signal activates (i.e., turns on and off) a ringoscillator 308. A center frequency of the ring oscillator 308 istunable. A ring oscillator 308 is used for its fast start-up time andbecause phase noise is not an important requirement in the UWBtransmitter 300.

An oscillating signal from the ring oscillator 308 and the triangularpulse from the pulse shaping circuit 304 are both fed into a mixer 310.The mixer 310 provides an upconversion of the triangle shape to thecenter frequency defined by the ring oscillator 308. The antennaadaptation circuit 312 provides an interface with the antenna.Requirements for the antenna adaptation circuit 312 include differentialto single-ended conversion, impedance adaptation, and filtering.

As described below, three of the components of the UWB transmitter 300are based on timing generation. The idea is to generate timing byintegrating charges into a defined capacitor and comparing the resultingvoltage over the capacitor to a reference voltage. A timing generationcircuit 400 is shown in FIG. 4. The timing generation circuit includesan integrator 402, a capacitor 404, and a comparator 406.

The pulse position modulator 302 uses the timing generation circuit 400.Based on the charge integrator described above, two different timingscan be generated by switching between two capacitor values using a datasignal. A precise timing difference can be defined by tuning thecapacitor value and the current. FIG. 5 depicts an example circuitdiagram of the pulse position modulator 302.

The time difference between two pulse positions will be given by thedifference in load capacitance (Cload) at the integration node A. Thatis: $\begin{matrix}{{\Delta\quad T} = {T_{2} - T_{1}}} \\{= {\frac{V_{ref}( {C_{par} + C_{1} + C_{2}} )}{I} - \frac{V_{ref}( {C_{par} + C_{2}} )}{I}}} \\{ \Rightarrow{\Delta\quad T}  = {\frac{V_{ref}}{I}( {C_{par} + C_{1}} )}}\end{matrix}$

In this design, C1 and the parasitic capacitance of the transistors(Cpar) are fixed in the design, so tuning the reference current (Iref)can modify the time difference. Typical values for this design are:Vref=  1 V Cpar= 200 fF C1= 300 fF (C2= 120 fF)For example, in order to obtain a time difference of 10 ns a current of50 uA is required.

The reset node B serves to bring the integration node A back to zero.Once the reset signal is set to zero (falling edge), the integrationnode A starts to rise, and once the integration node A reaches thereference voltage (Vref), the integration node A generates theoutput-enabling signal. The integration node A continues to charge andshuts the Pmos current source off, avoiding extra current consumption.Then, at the rising edge of the reset signal, the Nmos transistor shortsthe integration node A to ground, and the integration node A is readyfor a next integration. However, an extra Pmos transistor, controlled bythe complementary reset signal, shuts the current source off. This isessential since the current source is directly connected to ground atthat instant. The next falling edge instant releases the integrationnode A and switches the current source back on for the next integrationto occur. The essential voltages are shown in FIG. 6.

The pulse shaping circuit 304 also uses the timing generation circuit400. It is important to shape the pulse in order to avoid out of bandemissions. Indeed, if square-wave pulses are used, the harmonics of thesquare envelope give rise to unwanted lobes next to the spectrum of thesignal. These lobes can be attenuated or even suppressed by applying asmooth envelope to the pulse. The integration method described above isused to apply the smooth envelope to the pulse. This integration methoddoes not require extra special components, like step recovery diodes anddoes not consume much energy. The idea is to create triangle shapes bycharging and discharging a capacitor. The shape can then easily bemodified by playing with the charging current, the capacitor load, orthe reference voltage in the comparator. FIG. 7 depicts an examplecircuit diagram of the pulse shaping circuit 304.

The falling edge of the reset signal lets the I+ current flow into theload capacitor (Cload) by opening transistor M1 and closing transistorM3. Once the voltage at node A reaches the reference voltage (Vref), thecomparator opens transistor M4 and the same but inverted signal closestransistor M2. The load capacitor is connected to the Nmos currentsource and the load capacitor is discharged via the I− current. Thevoltage at node B results from this charging/discharging processproducing a triangular signal. Similar to the pulse position modulator302, the next rising edge of the reset signal brings the voltage back tothe initial state. The essential signals are shown in FIG. 8.

In order to vary the pulse duration, which has the effect of varying thebandwidth of the signal, a switch capacitor array may be used as Cload.A configuration signal fixes the capacitor value of Cload. For instance,a three-bit configuration signal can be used, fixing the capacitor valuebetween 200 fF and 550 fF (including the 150 fF parasitic capacitance)by steps of 50 fF. The duration of the triangle can be further tuned byadjusting the reference current (Iref). MOS capacitors may be used dueto their compatibility with standard logic processes. Since the voltageacross the resistor ranges between 0 and 1 V, Pmos transistors are usedto keep the capacitor value constant in this voltage range.

The ring activation circuit 306 also uses the timing generation circuit400. The goal here is to avoid the operation of the ring oscillator whenno pulses need to be created. An enabling signal is used andsynchronized to the shaping circuit. This starting event is easy toobtain since it is directly the pulse position modulator 302 output.However a stopping event must be created when the triangle duration isover. Since the triangle duration is tunable, using the same capacitorvalue as for the pulse shaping circuit 304 and a double current valuewill result in a duration that is equal to the triangle. A NANDoperation is used to build the ring enabling signal. FIG. 9 shows theresulting waveforms.

For obtaining the UWB communications signal, the oscillating signal isshaped by the triangle signal, i.e., the two signals are mixed. Thecircuit used for this operation is shown in FIG. 10. The operation issimilar to that of a traditional mixer. The transistor M1 is controlledby the triangular signal. This signal (0→1 V) modulates the tail currentsource following the quadratic law of a MOS transistor in its trioderegion. The resulting current flows in either branch of the differentialpair depending on the ring oscillator differential signal. The resultingvoltage at the output is an upconverted quadratic pulse shape. Ofcourse, the single ended voltage at each side of the output contains astrong common mode signal. However, either the differential output canfeed directly a differential antenna, or the output can be converted toa single ended signal by means of a balun. In order to reduce powerconsumption, an output buffer is avoided and the mixer is designed tofeed a 50 Ohm load. The different signals are shown in FIG. 11.

As can be seen in FIG. 11, the pulse starts once the triangular waveformreaches the threshold voltage (VT) of the transistor. The tunability ofthe frequency range is provided by the ring oscillator. The goal torange from 3 GHz to 5 GHz is achieved. The tunability of the bandwidthis from 500 MHz up to 2 GHz. These two extreme cases are shown in FIG.12. The bandwidth can be tuned to any value between these two extremes.

A maximum output power of −10 dBm is obtained. This output power can befurther tuned down to any lower value by either reducing the biasingcurrent of the output buffer or by reducing the triangular peak value.Reducing the output power makes sense if the power consumption of thesystem is dominated by the output stage. However, in this currentversion, 90% of the power consumption comes from the ring oscillator.Reducing the output power will help to save only a few percent of thepower consumption.

The overall power consumption is 10 mW during active mode and about 100uW during the sleep mode. The energy necessary to transmit one pulse is100 pJ for the long pulses (500 MHz) and 40 pJ for the short pulses (2GHz). The power consumption is divided as follows: Pcons= 8.5 mW for thering oscillator (85%) Pcons= 0.5 mW for the control circuits  (5%)Pcons=   1 mW for the mixer (10%)

An example chip layout of the UWB transmitter is shown in FIG. 13.

FIG. 14 depicts an example receiver. For the receiver, the basicprinciple is to shift data processing to the analog domain. The basicprinciple is then to implement a correlation receiver in the analogdomain and convert the result of the correlation to the digital domainto make the decision. The advantage is to allow a low frequency samplingat the ADC.

In this implementation, the accuracy on timings B, C, and D is not tootight since they are processing low frequency signals (ideally, theintegrator output is a DC signal). However, this architecture can beviewed as switching the high frequency timing constrains of the ADC intoa precise timing for the template generation (signal A). A small driftin the template signal with respect to the received signal degrades thecorrelation result. For example, assuming a 5th order gaussian asincoming pulse, a drift of 35 ps between the template signal and thereceived signal is sufficient to produce a wrong correlation value atthe output.

This problem with drift is shown in FIG. 15. The first dotted curve isthe received UWB signal, the second dotted curve is the template (atime-shifted version of the pulse), and the plain curve is thecorrelation function. The triangle shows the correlation value for thisparticular time-shift.

To overcome this stringent timing constraint on the template generationtiming (signal A), a correlation in quadrature can be used as depictedin FIG. 16. This type of reception technique shows a clear advantage ifthe received signal features an oscillating carrier. In that particularcase, the quadrature signal has the same pulse envelope, but has acarrier in quadrature inside. Different shapes for the envelope can beused to modulate the carrier. However, the shape of the transmittedpulse may define the signal bandwidth. A simple square shape shows toomuch side-lobe power, whereas a gaussian shape is too complicated togenerate with an analog circuit. Therefore, the triangular pulse shapeshows interesting advantages both in implementation simplicity andspectrum efficiency. The triangular shape is then preferably chosen asthe transmitted pulse.

The optimum receiver should ideally correlate the incoming pulse withits replica, being then the triangular pulse. However, using a squareshape as a template in the receiver still shows advantages compared to atriangular pulse:

-   -   lower implementation complexity;    -   correlation error due to a time shift between Tx and Rx clock        decreases slower with a rectangle than with a triangular shape;    -   rectangle will better capture the close distortion of the pulses        due to the channel; and    -   loss of correlation energy with the rectangle is negligible        compared to the correlation with a triangle.

In conclusion, the final architecture looks like a traditional directdown-conversion receiver where the local oscillator (LO) is duty cycledto produce pulsed I and Q LO signals. However, a substantial differencewith traditional system remains on the use of an analog integration inorder to bring a low frequency signal at the ADC input.

It should be understood that the illustrated embodiments are examplesonly and should not be taken as limiting the scope of the presentinvention. The claims should not be read as limited to the describedorder or elements unless stated to that effect. Therefore, allembodiments that come within the scope and spirit of the followingclaims and equivalents thereto are claimed as the invention.

1. A method for generating an ultra-wideband communications signal,comprising in combination: generating a piecewise linear ultra-widebandbaseband signal having at least one pulse, wherein the baseband signalis generated based on an inputted data signal; generating a carrier tonehaving a carrier frequency suitable for wireless transmission; andupconverting the baseband signal with the carrier tone to the carrierfrequency, thereby generating the ultra-wideband communications signal.2. The method of claim 1, wherein the at least one pulse has a durationand the carrier tone has a period, and wherein the duration is longerthan the period.
 3. The method of claim 1, wherein the ultra-widebandbaseband signal comprises a plurality of pulses separated from eachother in time.
 4. The method of claim 1, wherein the ultra-widebandcommunications signal has a bandwidth of at least one of at least 500MHz and at least 20% of the carrier frequency.
 5. The method of claim 1,wherein the ultra-wideband communications signal has a center frequencywithin one of a first range of 3-5 GHz and a second range of 6-10 GHz.6. The method of claim 5, wherein the ultra-wideband communicationssignal has a bandwidth of maximum 2 GHz if the center frequency iswithin the first range and maximum 4 GHz if the center frequency iswithin the second range.
 7. The method of claim 1, further comprisingtuning at least one of shape and bandwidth of the at least one pulse. 8.The method of claim 1, further comprising tuning the carrier tonefrequency.
 9. The method of claim 1, wherein generating a carrier toneincludes switching an oscillator off between two pulses.
 10. The methodof claim 9, wherein the oscillator is a ring oscillator.
 11. The methodof claim 1, wherein the at least one pulse is a triangular pulse.
 12. Amethod for interpreting a received ultra-wideband communications signal,the signal having a center frequency in a radio frequency domain,comprising in combination: generating a plurality of local signaltemplates, wherein the plurality of local signal templates arephase-shifted with respect to each other, are synchronized with thereceived ultra-wideband communications signal, and have substantiallythe same center frequency; correlating the received ultra-widebandcommunications signal with each of the local signal templates in ananalog domain, thereby obtaining an ultra-wideband baseband signal foreach correlation; and interpreting the ultra-wideband baseband signalsto generate a data signal.
 13. The method of claim 12, whereincorrelating the received ultra-wideband communications signal with eachof the local signal templates in an analog domain comprises: mixing thereceived ultra-wideband communications signal with each of the localsignal templates to obtain mixed signals; and integrating each of themixed signals to obtain the ultra-wideband baseband signals.
 14. Themethod of claim 12, wherein generating a plurality of local signaltemplates includes switching a ring oscillator on and off.
 15. Themethod of claim 12, wherein generating a plurality of local signaltemplates comprises for each signal template: generating a piecewiselinear ultra-wideband baseband signal comprising at least one pulse,wherein the baseband signal is generated based on the receivedultra-wideband communications signal; generating an upconversion tonehaving an upconversion frequency substantially equal to the centerfrequency of the received ultra-wideband communications signal; andupconverting the baseband signal with the upconversion tone, therebygenerating the signal template.
 16. The method of claim 12, whereingenerating a plurality of local signal templates includes generating twophase-shifted local signal templates, wherein a first local signaltemplate is shifted substantially 90° from a second local signaltemplate.
 17. The method of claim 12, wherein generating a plurality oflocal signal templates includes generating three local signal templateshaving a phase shift of approximately 120° between each other.
 18. Adevice for generating an ultra-wideband communications signal,comprising in combination: a signal generator for generating a piecewiselinear ultra-wideband baseband signal having at least one pulse, whereinthe baseband signal is generated based on an inputted data signal; anoscillator for generating a carrier tone having a frequency in the RFdomain; and a mixer for upconverting the baseband signal with thecarrier tone to the RF domain, thereby generating the ultra-widebandcommunications signal.
 19. The device of claim 18, wherein the signalgenerator has a pulse position modulator and a pulse shaping circuit.20. The device of claim 18, wherein the oscillator is a ring oscillator.21. The device of claim 18, further comprising a timing circuit forswitching off at least one of the signal generator and the oscillatorbetween two pulses.
 22. A device for interpreting a receivedultra-wideband communications signal, the signal having a centerfrequency in the RF domain, comprising in combination: a signalgenerator for generating a plurality of phase-shifted local signaltemplates having substantially a same center frequency as the receivedultra-wideband communications signal; a timing circuit for synchronizingthe local signal templates with the received ultra-widebandcommunications signal; an analog correlator for each local signaltemplate, wherein the analog correlator correlates the receivedultra-wideband communications signal with each local signal template,thereby obtaining an ultra-wideband baseband signal per correlator; andinterpretation means for deriving a data signal from the receivedultra-wideband baseband signal.
 23. The device of claim 22, wherein eachof the analog correlators includes a mixer and an analog integratordownstream of the mixer, wherein the received ultra-widebandcommunications signal and one of the local signal templates provideinputs to the mixer.
 24. The device of claim 22, wherein the signalgenerator has a ring oscillator, and wherein the timing circuit switchesthe ring oscillator on and off in synchronization with the receivedultra-wideband communications signal.
 25. The device of claim 22,wherein the device includes two analog correlators, and wherein thesignal generator includes a phase shifter for generating a second localsignal template that is shifted substantially 90° from a first localsignal template.
 26. The device of claim 22, wherein the device includesthree analog correlators, and wherein the signal generator maintains aphase shift of approximately 120° between the signal templates suppliedto each of the analog correlators.
 27. The device of claim 22, whereinthe interpretation means includes a baseband detection circuit fordetecting pulses in the ultra-wideband baseband signal.
 28. The deviceof claim 22, wherein the timing circuit includes at least one capacitor,a current source for supplying current to the at least one capacitor,and a comparator for comparing voltage on the at least one capacitorwith a reference voltage, and wherein the comparator provides an outputwhich forms a timing signal.
 29. The device of claim 28, wherein thedevice includes a first capacitor and a second capacitor parallel to thefirst capacitor, and wherein the device includes a data signalcontrolled switch that is operable to connect and disconnect the firstcapacitor from the comparator.
 30. The device of claim 28, wherein thetiming circuit further includes a reset node for resetting the timingcircuit by discharging the at least one capacitor.
 31. The device ofclaim 28, wherein the timing circuit further includes a feedback of theoutput of the comparator, wherein the feedback is used for dischargingthe at least one capacitor.